The present invention is generally directed to the treatment of materials such as earthen materials and, in particular, to a method and apparatus for tilling base material in a manner that forms a mixing area where additives may be applied to and mixed with the tilled base material. The method and apparatus are particularly suitable for use in treatment of a leach pad of a precious metals mining operation.
In many in-situ material treatment processes, it is desirable to reduce compaction of the material. Uncompacted material requires less energy and less additives to process, resulting in decreased environmental impact. Reduced compaction increases the permeability and effective surface area of the material, which can enhance the effects of additives that are often applied to the material in-situ to cause a change in the material. A need exists for improved methods and devices for reducing compaction and treating materials in-situ. Contaminated soil remediation and chemical leaching operations are two processes that can benefit from reduced compaction. Because opportunities for particularly significant economic and environmental benefits exist in a leaching process of a precious metals mining operation, the present invention will be described with reference to that process.
Mining for precious metals, such as, for example, gold, platinum, silver, and copper, commonly involves a leaching process that is used to extract these metals from a low grade raw ore. In such a mining operation, the ore is typically collected on a heap leach pad built on the surface of a relatively flat land area many acres in size. FIG. 1 is a side elevational view of a leach pad 10 being constructed in accordance with the prior art. FIG. 2 is a diagrammatic illustration of a multi-lift leach pad in accordance with the prior art. With reference to FIGS. 1 and 2, the leach pad 10 is constructed on a basin 11 having a crowned surface covered by a liner. The leach pad 10 is supplied with ore 12 brought in by large dump trucks 13 to form a layer of ore called a lift 14, typically having a depth of between 9 and 50 feet. After the lift 14 is formed, a liquid leaching agent is applied to the upper surface of the lift 14, usually by a sprinkler system (not shown). The leaching agent percolates through the lift 14 and dissolves or otherwise binds to metals in the ore. The laden leaching solution, often called the leachate or pregnant leachate, is contained by the liner and is collected at the perimeter of the leach pad 10 for transportation to a refining facility where the metals are chemically extracted from the laden leaching solution. When the concentration of metals in the leachate decreases to a certain level, a fresh lift of ore 15 (FIG. 2) is then deposited over the spent ore and the process is repeated. Multiple lifts are formed so that the leaching agent continues to percolate through the lower lifts to maximize the yield of the ore.
It is important that the lift be evenly permeable so that the leaching agent can percolate completely throughout the lift. However, the permeability of the lift decreases due to the way in which it is built. In building the lift, the dump trucks 13 may deliver, for example, 38,000 loads of raw ore with each load of raw ore weighing between 75 and 300 tons. The lift 14 is compacted by the weight of the trucks 13 as they drive over it to deliver each load of ore 12, as shown in FIG. 1. The compaction of the lift 14 is greatest near the upper surface of the lift and decreases with depth. Substantially all truck compaction is found in the top six to nine feet of the lift. Ideally, the ore would consist entirely of clean gravel that remains highly permeable, even when compacted. However, ore commonly includes fines, silts, and clay that form a less permeable matrix with the gravel when compacted. Poor permeability inhibits the free flow of the leaching agent through the lift and lessens the yield of the ore.
A ripper is typically used to break up the ore at the upper surface of the lift. A ripper is a bulldozer that drags a shank through the upper surface of the lift. FIG. 3 shows a prior-art ripper 16 with its shank 17 retracted. FIG. 2 shows the path followed by the ripper 16. Rippers have proven only partially effective in reducing compaction because they are typically unable to extend their shanks deeper than 60 inches. Additionally, known rippers have shanks that produce only a narrow path of ripped lift material, typically less than 6 inches wide. Because the shank 17 is narrow, the ripper 16 usually leaves pillars or cells of compacted ore between adjacent paths of the shank 17, resulting in less than optimal permeability of the leach pad. The liquid leaching agent will follow the path of least resistance as it filters through the lift. The compacted cells and pillars form flow channels 18 between them that shunt the flow of the leaching agent and can prevent it from percolating through entire sections of the lift. Hydrodynamic effects of flow channels can also cause fines and silts to form dams and lenses within the lift that further hinder leachate dispersion. Lenses (subsurface ponds) and dams reduce ore yield by producing a shadowing effect that leaves dry spots in the lift. Lenses and dams have lesser effect in shallow pads (9-20 ft. deep) because there is less material to be shadowed. In a taller lift, lenses formed near the top of the lift will shadow larger amounts of ore. On the other hand, the effects of flow channeling (in the absence of dams and lenses) are typically more pronounced in shallow pads (9-20 ft. deep) due to shorter soak times and the shorter distance from the top of the lift where the leaching agent is applied, down to the relatively dead material at the bottom of the leach pad.
To improve the permeability of the leach pad, fine ores are sometimes treated with an agglomerating treatment prior to being deposited onto the leach pad so that the fine particles will agglomerate into larger clumps that are more loosely organized. Agglomerated material tends to inhibit the formation of lenses and dams within the lift because it has drainage properties that are similar to gravel. However, agglomerated material is not very resistant to compaction caused by the weight of delivery trucks driving over the agglomerated material after it is deposited on the lift. Consequently, a need exists for improved methods of agglomerating that do not subject the agglomerated materials to compaction forces.
Machines have been proposed for tilling compacted soil as a step in environmental remediation of contaminated soil. For example, U.S. Pat. No. 5,639,182 of Paris describes a method for treating soil in which a treatment material is first spread over the surface of the soil then tilled into the soil by a mixing apparatus. Paris does not disclose the use of the mixing apparatus or process in a precious metals mining operation for extracting metals from ore. The mixing apparatus includes a vertically-oriented endless cutter that is towed behind a tractor over the soil area to be treated. The endless cutter is extended into the soil to a cutting depth and activated to drive the treatment material down into the hard soil and to provide a mixing effect. The soil and treatment material is driven down and around the lower end of the cutter and back up to the soil surface at a location distal of the tractor. The mixed soil and treatment material may be moved to the side of the machine by a lateral conveyor. Because the treatment material is spread on the surface of the soil before mixing, the mixing apparatus may not always mix it evenly throughout the cutting depth. Furthermore, the teeth of the endless cutter are shaped and sized so that fines would tend to remain at the cutting depth without being drawn back to the surface for a more thorough mixing with the treatment material. It would also be ineffective for agglomeration of fines into small clumps because the cutter remains in contact with the soil throughout the mixing process, which would tend to break up agglomerated clumps.
U.S. Pat. No. 5,830,752 of Bruso describes a soil treatment apparatus including a backhoe-type tractor having a boom-mounted endless cutter for tilling soil and an injection system for applying a liquid remediation agent to the soil as it is tilled. Bruso does not disclose the use of the soil treatment apparatus in a precious metals mining operation. Nor does Bruso disclose the use of the apparatus in a leach pad (mining, environmental, or otherwise). The injection system described by Bruso is located along the length of the cutter opposite the tractor for spraying a liquid into the excavated soil. The cutter is much narrower than the width of the tractor, which requires the cutter to be repeatedly dipped and dragged through the soil in multiple swipes to cover the entire width of the path traveled by the tractor. This precludes continuous movement of the tractor along the surface of the soil, results in cutter paths that fan out from the tractor, and tends to lift the tractor each time the cutter is dipped into the soil, all of which diminish the capacity of the apparatus. Similar to the ripper described above (FIG. 3), the apparatus of Bruso tends to leave compacted cells and pillars between adjacent swipes of the cutter. The fanning-out of the swipes amplifies this effect at the distal end of the swipes. In addition, the cutter is positioned to perform an xe2x80x9cover-cuttingxe2x80x9d operation, meaning that the lower end of the cutter is tilted away from the direction of movement of the machine. Over-cutting results in the leading and trailing sides of the cutter being in continuous contact with the soil. For this reason, the apparatus would be ineffective for agglomeration of fines. The cutter also has a tendency to drag tilled soil around the cutter multiple times, which can actually create fines.
Thus, there exists a need for a more effective method and apparatus for reducing compaction in a leach pad and other base materials. Significant opportunities for increasing the yield of a leach pad and for improved in-situ processing also exist.
In accordance with the present invention, a tilling apparatus is used for breaking up compacted base materials, for example, compacted ore in a leach pad of a precious metals mining operation. The tilling apparatus includes a mobile carrier onto which a tilling head is mounted. The tilling head preferably includes a cutter comprising an endless belt constructed of multiple, linked tilling bits that have teeth that extend across substantially the entire width of the tilling head. The tilling head preferably includes a hydraulic drive motors that operate to drive the endless belt around the tilling head. Thy hydraulic drive motors are powered by the carrier and are coupled to gear boxes, transmission chains, and other mechanical components of the tilling head. Alternatively, the cutter could be powered by other drive means. In operation, the tilling head is extended into the base material to a tilling depth so that a portion of the cutter forms a cutting face of the tilling head that engages and loosens the base material. The tilling head deposits the loosened base material opposite the cutting face.
The tilling apparatus may include an additive delivery system for applying an additive to the loosened base material while it is being deposited or immediately before it is deposited by the tilling head. For example, the additive delivery system may include spray nozzles for applying liquid, powdered, or gaseous additives to the loosened base material as it is thrown from the tilling head. In a preferred embodiment, a control unit for controlling the tilling head may also include an additive management system for controlling the composition and quantity of the additive being applied to the loosened base material. The rate of application of additives can then be automatically controlled based on the grade and size of loosened ore, the type of additive, the velocity of the carrier, the working load on the tilling head, and the speed of the cutter, for example.
As used herein, the term xe2x80x9cbase materialxe2x80x9d means any solid or semi-solid material through which a tilling head operating in accordance with the present invention can be conveyed for tilling or mixing the material. Base material may include, without limitation, ores, soil, leach pads, sludge, trash, industrial waste materials, marshland, swamps, riverbeds, and sea floors. The term xe2x80x9cadditivexe2x80x9d, as used herein, shall mean any liquid, solid, or gaseous substance that may be applied to the base material as part of a process of chemically and/or physically transforming the base material. Additives can be used for extraction of precious metals and/or for remediation of contaminated soils. Additives may include chemical leaching agents for extraction of metals, including precious metals such as gold, platinum, silver, and copper. Examples of common chemical leaching agents include aqueous solutions of cyanide salts (for gold extraction), including sodium cyanide solution and potassium cyanide solution; dilute sulfuric acid (for copper extraction). Other additives include bioleaching agents such as thiobacillus ferroxidans and leptosprillium ferroxidans bacteria, and agglomerating agents such as milk of lime, fly ash, and portland cement.
The tilling head can be positioned so that the cutting face undercuts the base material to facilitate lifting of the loosened base material over the tilling head and depositing it on a pile formed away from the tilling head opposite the cutting face. So formed, the pile has a slope that is generally inclined at an angle of repose of the loosened base material. This slope serves as a mixing area where additives can be mixed with the base material as it tumbles down the slope. Preferably the tilling head deposits the loosened base material far enough away from the tilling head so that the loosened base material, once deposited, remains substantially undisturbed by the tilling head, even at the tilling depth. A chute can be mounted on the tilling head so that it extends away from the top of the tilling head for carrying the tilled base material far enough away from the tilling head to attain this result.
As used herein, the term xe2x80x9cangle of reposexe2x80x9d shall mean the steepest slope angle measured from the horizontal that the loosened base material is capable of forming when dropped onto a pile. This definition may vary slightly from accepted definitions of static angle of repose and dynamic angle of repose, which indicate specific test conditions not necessarily applicable to the present invention. The angle of repose is a function of the size and shape of the material that forms the pile, as well as other factors, such as moisture content and the method of forming the pile. Ore of a typical precious metals leach pad has an associated angle of repose ranging between approximately 36xc2x0 and approximately 39xc2x0. However, finer and coarser ores and other base materials may have associated angles of repose ranging from approximately 25xc2x0 or less to approximately 45xc2x0 or more.
Additional aspects and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment thereof, which proceeds with reference to the accompanying drawings.